Silicon single crystal, one of the crystals used as semiconductor device substrates, is primarily manufactured by the Czochralski method (hereinafter abbreviated as “CZ method”).
When crystals are manufactured by the CZ method, a crystal manufacturing apparatus as shown in FIG. 4 is, for example, used. The manufacturing apparatus has, for example, members for melting a raw material polycrystal such as silicon and a mechanism for pulling monocrystallized silicon, and these members and the mechanism are accommodated in a main chamber 11. A pulling chamber 12 extending upward is connected from a ceiling portion of the main chamber 11, with a mechanism (not shown) provided on top thereof for pulling a crystal 4 with a wire 10.
Within the main chamber 11, a crucible 5 is arranged for containing a melted raw material melt 6, with the crucible 5 supported by a shaft 9 so as to be free to rotate and move up and down with a drive mechanism (not shown). To compensate for decline in melt level as a result of pulling of the crystal 4, the drive mechanism for the crucible 5 is designed to raise the crucible 5 as much as the melt level declines.
A heater 7 for melting raw material is arranged so as to surround the crucible 5. A heat-insulating material 8 is provided outside the heater 7 so as to encircle the heater 7, thus preventing direct radiation of heat from the heater 7 to the main chamber 11.
A lump of raw material is accommodated in the crucible 5 arranged within such a crystal manufacturing apparatus followed by heating of the crucible 5 with the heater 7, thus melting the lump of raw material in the crucible 5. The crystal 4 of a desired diameter and quality is grown below a seed crystal 2, fastened with a seed holder 1 connected to the wire 10, by allowing the seed crystal 2 to be immersed into the raw material melt 6 resulting from melting of the lump of raw material and then pulling the seed crystal 2. At this time, so-called “necking” is performed in which a neck portion 3 is formed by narrowing the diameter to roughly 3 mm after the seed crystal 2 is immersed into the raw material melt 6, followed by thickening of a crystal until a desired diameter is reached and then pulling of the dislocation free crystal.
The so-called MCZ method (Magnetic field applied Czochralski Method), an improved version of the CZ method, is also known recently. With the MCZ method, a magnetic field is applied to a raw material melt, suppressing thermal convection of the raw material melt and thus manufacturing a crystal. While large diameter silicon single crystals of eight inches or more in diameter are recently in demand, the MCZ method capable of suppressing thermal convection of the raw material melt is effective for manufacturing such large diameter silicon single crystals.
Here, the heater 7 for manufacturing a crystal used in the aforementioned CZ and MCZ methods is cylindrical shape as shown in FIG. 1 and primarily made of isotropic graphite. In the direct current type, currently in vogue, two terminal portions 7b are provided, with the heater 7 supported by the terminal portions 7b. A heat generating portion 7a of the heater 7 has slits 7c provided at several to several tens of locations for efficient heat generation. It is to be noted that the heater 7 generates heat mainly from individual heat-generating slit portions 7d—the portions between the lower end of slits extending from the top and the upper end of slits extending from the bottom.
The crucible must be naturally used larger in size to manufacture large diameter crystals in demand recently at low cost. As a result of upsizing of the crucible, structures around the crucible such as the heater have been upsized. Upsizing of the heater has led to problems to deformation of the heater while in use for crystal manufacture due to heater's own weight and ununiform heat distribution, and further because of interaction between magnetic field and current in the case of magnetic field application as in the MCZ method. Deformation of the heater while in use in crystal manufacture changes the distance between the heater's heat-generating portion and the crystal ingot or the melt, thus changing the heat distribution. This in turn gives rise to ununiform temperature within the raw material melt, resulting in detrimental effects such as hindrance of monocrystallization of the crystal manufactured and unstable quality.
As a countermeasure thereof, dummy terminals are commonly attached to the heater in addition to the two terminal portions, thus supporting the heater by three or more portions (e.g., Japanese Patent Publication No. 7-72116). If the heater is supported by the terminal portions alone, it is supported only at two locations, resulting in easy deformation at those portions with no terminals. This deformation takes place at the upper portion of the heater in such a manner that those portions with terminals expand in diameter while those portions without terminals diminish in diameter. Providing dummy terminals at the portions with no terminals has been effective to a certain extent to prevent such a heater deformation.
However, the heater is divided by slits, making it impossible to completely prevent deformation by provision of terminals and dummy terminals alone. In the case of a larger diameter or taller heater in particular, it is difficult to suppress deformation. In addition to the above, this method led to other problems such as heat loss from the dummy terminal portions and further more complex mechanical construction.
Further, in the MCZ method for manufacturing large diameter crystals while applying a current magnetic field, deformation is caused not only by heater's own weight and thermal expansion but also electromagnetic force resulting from interaction between heater current and magnetic field. This electromagnetic force is considerably strong, making it difficult to avoid heater deformation even if the heater is supported by the entire lower end.
Accordingly, a method is proposed in the MCZ method that prevents deformation by using a doubled-structured heater consisting of slitted inner and outer heaters and apply direct currents in different directions in respectively, thus suppressing electromagnetic force resulting from interaction between heater current and magnetic field (e.g., Japanese Patent Application Laid-Open Publication No. 9-208371). The aforementioned method, although being effective to a certain extent for preventing deformation due to electromagnetic force, results in problems such as considerably increased cost due to more complex mechanical construction and larger deformation, conversely, due to heater's own weight.
Meanwhile, an attempt is being tried out to change the heater material from isotropic graphite to a stronger and lighter material such as carbon composite. However, this method leads to problems such as unstable heat generation, higher heater material cost and further lower purity of crystal manufactured.